Various types of power systems use a switch, such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), to isolate circuit nodes or elements, for example to isolate a voltage bus from a load, in the event of fault, such as a short circuit or overload condition. In a typical configuration of this type, the switch is coupled in series between the bus and the load. The switch is normally on and, in the event of a short circuit or overload condition, the switch turns off to isolate the circuit nodes. The isolation device and associated circuitry may be designed to protect against a short circuit on the bus and/or in the load or various other types of fault conditions. The bus may be provided by a power supply or converter, a plurality of parallel converters, a motor driver, or a UPS, as just a few examples.
One such power system is described in a U.S. Pat. No. 6,615,146 entitled “Failure Detection of an Isolation Device with PFA Signal Generation in a Redundant Power Supply System” to Malik et al., in which a MOSFET isolation device is coupled in series within a power supply of a redundant power supply system. When a short circuit occurs on a voltage input line of the power supply, Vin, the switch is turned off to isolate the power supply from the bus.
In the '146 circuit, a predictive failure analysis (PFA) signal is generated when the isolation device fails, thereby advantageously signaling the presence of a failure of the isolation device. More particularly, if the gate, drain and source of the switch all short together, then the voltage on the gate, VMONITOR, will be at the same value as the Vout voltage, causing the PFA signal to go low, thereby indicating a failure of the switch.